But they dont work too well as a battery charger as the open circiut voltage on these switch mode supplies is just that, 12v. But you can change that

if you look at the yellow wire to the circiut board, somewhere along that track it has to lead to two resistors, in a T config. One goes to ground, one to the 12v. and the connection between goes to the regulator chip, (power supply controller) 
If you change one of these resistances values, you can set your output voltage higher. Typicaly you would set it to 13.8v